1. Field of the Invention
The present invention generally relates to data processing and more particularly to utilizing logical fields with conditional constraints in abstract queries.
2. Description of the Related Art
Databases are computerized information storage and retrieval systems. A relational database management system is a computer database management system (DBMS) that uses relational techniques for storing and retrieving data. The most prevalent type of database is the relational database, a tabular database in which data is defined so that it can be reorganized and accessed in a number of different ways. A distributed database is one that can be dispersed or replicated among different points in a network. An object-oriented programming database is one that is congruent with the data defined in object classes and subclasses.
Regardless of the particular architecture, in a DBMS, a requesting entity (e.g., an application or the operating system) demands access to a specified database by issuing a database access request. Such requests may include, for instance, simple catalog lookup requests or transactions and combinations of transactions that operate to read, change and add specified records in the database. These requests are made using high-level query languages such as the Structured Query Language (SQL) and application programming interfaces (API's) such as Java® Database Connectivity (JDBC). The term “query” denominates a set of commands for retrieving data from a stored database. Queries take the form of a command language, such as SQL, that lets programmers and programs select, insert, update, find out the location of data, and so forth.
Unfortunately, generating queries using SQL may require a detailed understanding of the possibly complex physical layout of the underlying database and interpretation of cryptic field names. For some applications, to facilitate the query building process, an abstraction model may be utilized that, in effect, hides some of the complexities of the underlying database physical layout from users. The abstraction model may include logical fields with recognizable names that map to corresponding physical fields of the underlying database. “Abstract” queries may be generated containing conditions based on the logical fields. Upon issuance, the logical fields of an abstract query may be mapped to corresponding physical fields to create a physical or “concrete” query. For some applications, abstract queries may be saved, allowing subsequent users to reuse the saved abstract queries without having to generate their own. The concepts of data abstraction and abstract queries are described in detail in the commonly owned, co-pending application Ser. No. 10/083,075, entitled “Improved Application Portability And Extensibility Through Database Schema And Query Abstraction,” filed Feb. 26, 2002, herein incorporated by reference in its entirety.
Abstract queries often contain a conditional element (e.g., a WHERE clause in a corresponding SQL statement) in which a logical field is used within an expression with a comparison operator. As described in the related application Ser. No. 10/083,075, logical fields are typically mapped to a particular physical field in a physical database. For instance, if the physical database was implemented as a relational database, a particular logical field would be mapped to a particular column within a relational table.
An advantage of abstraction is that users are able to create complex queries in order to retrieve desired results. Unfortunately, complex abstract queries translate to extremely elaborate physical queries that join numerous database tables in a complex manner to assemble the required result set. Such physical queries require a large amount of system resources to execute.
Generally, the DBMS includes a query optimizer component configured to determine the manner in which queries will be processed. The primary task of the optimizer is to determine the most efficient way to execute each particular query against a database. To this end, the optimizer typically determines an access plan for use in executing the query against the database. In general, the access plan contains low-level information indicating precisely what steps the system is to take to execute the query. Commonly, the access plan calls for the order in which tables are joined and the use of one or more indexes carefully designed to speed execution of the query. The inclusion or exclusion of joins between particular tables can have a tremendous impact on query performance.
Therefore what is needed is an improved process for constructing optimized abstract queries that can be converted into more efficient physical queries.